It is known to use special stiffening members for stiffening sheet metal, molded plastic articles, and the like, especially of car body parts. Generally, said stiffening members are flexible multi-layer body members which are adherent on their one side and may be composed of:
an actual stiffening layer, in particular one comprising a glass cloth bound by means of a thermosetting resin; and PA1 an adhesive layer, in particular one comprising a thermosetting resin. PA1 that the multi-layer body member, when applied to the surface to be stiffened, has a convex form so that it contacts the surface of the work piece first in its middle and thereafter progressively outwardly, whereby any bubble formation is avoided; and PA1 that, if multi-layer body members are used which comprise a protection foil to be drawn off, said protection foil is loosened from the adhesive layer in a marginal zone before the actual drawing-off, in order to allow thereafter a drawing-off by means of vacuum.
In the interest of a simple application of the multi-layer body members, the adhesive layer generally is made to have a tacky surface, so that the stiffening member, once applied, is sticking to its place until the heat treatment is accomplished.
However, this requires that the tacky surface of the adhesive layer is protected until the multi-layer body members are used in order to protect them from contamination and to allow stacking them.
According to the status of the art, such a protection may be performed by applying a protection foil, especially consisting of a suitably treated paper, to the multi-layer body member and removing said protection foil immediately before the application of the stiffening member.
One disadvantage of using stiffening members of this kind is that removing the protection foil requires a supplemental labour-intensive operation step which so far could not be accomplished mechanically. This was particularly unwelcome in the motor car industry where there is the trend for automation of all operation steps.
In order to avoid said supplemental operation step, it was proposed in the DE-OS 37 20 267 to accomplish said protection by covering the adhesive layer with a heat shrinking film which is regularly clearing said adhesive layer upon shrinking, e.g. by providing it with slits which widen to form openings upon heat curing thereby clearing a considerable portion of said adhesive layer.
However, this solution has the disadvantage that only a portion of the adhesive layer is cleared. For this reason the adhesion of the stiffening member is less than in the case of a full-surface bonding.
Ultimately, it was also proposed to use multi-layer body members which, instead of an adhesive layer having a tacky surface, comprise a layer of hot-melt adhesive that is not tacky at room temperature but becomes tacky under the influence of heat only.
Such multi-layer body members also have the disadvantage of a poor adhesion.
Another unsolved problem was that of applying the stiffening member, free from voids, to the surface to be stiffened. This is absolutely necessary, since any formation of bubbles between the adhesive layer and the surface to be stiffened is considerably lowering the stiffening effect due to reduction of the effective bonded area.